Optical pulse lasers have great potential for applications in various fields, such as optical communications, optical signal processing, laser surgery, biomedicine, optical diagnostics, two-photon microscopy, optical probing, optical reflectometry, material processing, etc. There are two main classes of optical pulse lasers, namely mode-locked lasers and Q-switched lasers. Mode-locked lasers can produce ultra-short optical pulses at high repetition rates, whereas Q-switched lasers are generally used for generating high-energy pulses at relatively low repetition rates.
As is known in the art, a mode-locked laser has multiple longitudinal modes that oscillate simultaneously with their relative phases locked to each other at fixed relationship generating uniformly spaced pulses. The longitudinal modes are defined by the effective path length of the laser resonator. In order to achieve mode locking, a mode-locking mechanism is required to synchronize the phases of the lasing modes so that the phase differences between all lasing modes remain constant. These optically phase-locked modes then interfere with each other to form optical pulses. Two broad classes of mode-locking schemes, active mode locking, and passive mode locking, are typically used and various methods and devices are known in the art for implementing such mode-locking schemes. U.S. Pat. Nos. 3,978,429; 4,019,156; 4,435,809; 4,665,524; 5,764,679; 5,802,084; and 5,812,308 provide examples of mode-locked lasers.
Active mode-locking schemes employ an intensity or phase modulator in the laser cavity operating at frequencies equal to the fundamental cavity frequency, or at an integer multiple or a rational multiple of the fundamental cavity frequency. An example of active mode locking is provided in U.S. Pat. No. 4,019,156.
In contrast, passive mode-locking schemes use at least one nonlinear optical element or device in the lasing cavity, or within a cavity external, but optically coupled, to the lasing cavity, that possess an intensity-dependent response to favor optical pulse formation over continuous-wave lasing. A passively mode-locked laser requires at least one nonlinear optical element as a mode-locker. A nonlinear optical element could possess properties such as amplitude nonlinearity (absorption as a nonlinear function of input optical intensity), Kerr-type (phase or refractive index as a nonlinear function of input optical intensity) nonlinearity, or a combination of both to facilitate mode locking. Amplitude nonlinearity could be provided by device such as a saturable absorber with a fast recovery lifetime in the order of picoseconds, such as the MQW semiconductor (amongst all available saturable absorbers, there are few which possess a fast response in the picosecond regime). Alternatively, Kerr-type nonlinearity, such as those implemented in the interferometric pulse addition method [see for example, Mark, 1989, or Ippen, 1989] and the Kerr-lens method (Kerr-focusing, self-focusing) [see for example, Spence, 1991, or Brabec 1992], could be used to provide an ultra-fast laser mode-locking mechanism. Although not a saturable absorber, the non-linear optical properties such as the Kerr effect, give an artificial “saturable absorber” effect, which has a response time much faster than any intrinsic saturable absorber.
A saturable absorber is a material that displays a change in its optical transparency dependent on the incident optical intensity in a specific operating wavelength region. In a linear regime, where the incident optical intensity is weak, the saturable absorber absorbs the incident light, resulting in attenuation of the optical intensity of the incident light. When the incident optical intensity is raised to a higher level, saturation of absorption occurs and absorption by the saturable absorber decreases, resulting in a decrease in attenuation of the optical intensity of the incident light. This kind of intensity-dependent attenuation allows the high intensity components of the pulse to pass through but not the low intensity components, such as the pulse wings, pedestals and background cw radiation. When a saturable absorber is placed in a lasing cavity, it will favor pulsing modes over cw modes. However, not all saturable absorbers are suitable for ultra-short-pulse mode-locking application. The important properties of a laser mode-locker are the saturation fluence, recovery time, and nonlinear/linear-absorption ratio. The saturation fluence will affect laser operating power level, which is limited by the device damage threshold. The recovery time limits the shortest achievable pulse width and the laser operating regime. For a given saturable fluence and recovery time, the laser could operate in one of four different operating regimes: cw lasing (without pulsing), Q-switching, Q-switched mode-locking, and cw mode-locking. A fast device recovery time, in picosecond and sub-picosecond regimes, is required for ultra-short pulse generation, whilst a slow recovery time, in the nanosecond regime, could give raise to Q-switching modes. However, a slow recovery time is also essential for self-starting of a mode-locked laser. Therefore, a mode locker, is a type-of saturable absorber that exhibits additional properties beneficial for functioning to mode lock a laser. A mode locker material, which is the functional material in a mode-locker element or device useful in laser configuration herein, should preferably possess both a fast and a slow recovery time in order to be used effectively in a pulsed laser operating in the picosecond and sub-picosecond regimes. There are many materials possessing nonlinear properties (such as saturable absorption) that do not possess the properties of a mode-locker. The CNT materials including layers containing SWNTs, or a combination of SWNTs and MWNTs exhibit mode-locker properties.
Passive mode-locked lasers are exemplified in U.S. Pat. Nos. 3,978,429 and 4,435,809. Hybrid mode-locked lasers which combine active and passive mode locking mechanisms are also known. An example of hybrid mode-locked laser is disclosed in U.S. Pat. No. 4,019,156.
Q-switching and self-starting (initiate pulsing) of lasers also employ non-linear optical materials and/or saturable absorbers. Passive Q-switched lasers are exemplified in: U.S. Pat. Nos. 4,191,931; 5,119,382; and 5,408,480.
The most commonly known saturable absorbers for laser mode-locking and Q-switching are materials such as an organic dye medium [see for example, Ippen, 1976] or a multi-quantum well (MQW) semiconductor device [see for example, Chemla, 1986, or Keller, 1992].
Organic materials such as dyes can exhibit a broadband absorption response over hundreds of nanometers. However, the use of dyes in laser configurations requires the use of mechanical elements such as nozzles, which are bulky and subject to mechanical malfunction and are not easily integrable with solid state lasers. At longer wavelength in the infrared region, particular at the telecom wavelength of 1550 nm, the available dye media are easily damaged by visible light, making it more difficult to handle such materials.
MQW (multiple quantum well)semiconductor devices require complex and costly fabrication systems, such as MOCVD (metal organic chemical vapor deposition) or MOVPE (metal-organic vapor-phase epitaxy, and may require additional substrate removal process. Furthermore, high-energy (4 MeV˜12 MeV), heavy-ion implantation is required to reduce the device recovery time (typically a few nanoseconds) to a few picosecond for laser mode-locking. The MQW saturable absorber can only be used in reflection mode, therefore requiring inclusion of an optical circulator, which increases the total device insertion loss. Additionally, MQW-based devices may require expensive hermetic packaging for long-tern environmental stability, and may not withstand high optical input powers. So far, no alternative material useful as a saturable absorber at 1550 nm has been found to challenge MQW-based saturable absorbers.
Thus, there is a need in the art for materials that exhibit non-linear optical properties and materials which function as saturable absorbers for use in laser and other optical device applications. This invention relates to the use of new saturable absorber materials, carbon nanotubes, and particularly single walled carbon nanotubes, for use in laser applications.
It has recently been reported that single-wall carbon nanotubes (SWNTs) exhibit saturable absorption [Y.-C. Chen, et al., 2002a and Y. Sakakibara, et al., 2003], and the potential application for such material as optical switches was proposed [Y.-C. Chen, et al., 2002a ; Y.-C. Chen, et al., 2002b; and Y. Sakakibara, et al., 2003]. International application WO03/034142 reports the saturable absorption properties of SWNTs and certain optical devices that include SWNTs. The Z-scan measurement technique which was used in the studies presented does not measure device response time. In separate studies, the recovery time of a thin layer containing SWNTs was measured to be <1 ps using pump-probe experiments [Y.-C. Chen, et al., 2002 a; Y.-C. Chen, et al., 2002b; and S. Tatsuura, et al., 2003]. Recently, a SWNT-based saturable absorber called “Saturable Absorber Incorporating NanoTube” (SAINT) was reported for use in optical noise suppression of ultrafast optical pulses in the picosecond regime [S. Y. Set, et al., 2003a].
Certain aspects of this invention have been reported. A passively mode-locked fiber laser using SAINT as a mode-locker was reported [S. Y. Set, et al. 2003b]. A Q-switched laser using SAINT as a Q-switch was reported [S. Y. Set, et al. 2003c].